1. Field of the Invention
Aspects of the present invention relate to a fixing device of an image forming apparatus, and more particularly, to a belt-type fixing device and an image forming apparatus having the fixing device.
2. Description of the Related Art
Image forming apparatuses, such as printers, copiers, scanners, multi-function machines, or the like, include fixing devices which apply heat and pressure to and semi-permanently fix developer images. These developer images are transferred onto printing media by transferring units generally known to those skilled in the art. Such fixing devices can be divided into roller-type fixing devices, which include heating rollers containing a heat source and pressing rollers which are biased toward the heating rollers to form nips, and belt-type fixing devices, in which fixing belts are used.
FIG. 1 is a sectional view schematically illustrating an example of a conventional belt-type fixing device 70. As shown in FIG. 1, the conventional belt-type fixing device 70 includes a pressing roller 10, a fixing belt 20 which rotates in contact with the pressing roller 10 and forms a nip N, a first roller 30 and a second roller 40 which guide the rotation of the fixing belt 20, a heat source such as a heating member 50 mounted in the first roller 30 to heat the fixing belt 20, and a sensor member 60 to control the temperature of the fixing belt 20.
The first roller 30 is disposed to face the pressing roller 10 with the fixing belt 20 disposed therebetween. The second roller 40 is spaced apart from the first roller 30 by a predetermined distance. The first and second rollers 30 and 40 support the fixing belt 20 to rotate the fixing belt 20 according to a regular cycle.
The heating member 50 includes a halogen lamp (not shown) contained therein. The heating member 50 is mounted in the second roller 40 along the rotational axis of the second roller 40. The sensor member 60 includes a thermostat, which is mounted in a central latitudinal portion of the fixing belt 20 to detect the surface temperature of the fixing belt 20 heated by the heating member 50.
The conventional belt-type fixing device 70 as described in connection with FIG. 1 applies pressure exerted by the pressing roller 10 and heat applied to the fixing belt 20 to a developer image transferred onto a printing medium P when the recording medium P passes through the nip N formed in a region in which the pressing roller 10 and the fixing belt 20 contact each other. Thus, the conventional belt-type fixing device 70 fixes the developer image onto the printing medium P.
When a printing medium P having a relatively large width dimension, such as an 8.5″×11″ sheet of paper, is used in the conventional belt-type fixing device 70 configured as described above, the width of the fixing belt 20 (i.e., the width in a direction substantially parallel to a rotational axis of the fixing belt 20) corresponds to the width of the printing medium P. Thus, the total amount of heat generated from the fixing belt 20 is evenly transferred onto the printing medium P, and there is no latitudinal variation in temperature distribution across the fixing belt 20. However, when a printing medium P having a relatively small width dimension, such as an envelope, is used in the conventional belt-type fixing device 70 configured as described above, the temperature of a portion of the fixing belt 20 through which the printing medium P passes (that is, the central latitudinal portion of the fixing belt 20) is normally reduced, but the temperature of opposite sides, i.e., opposite ends, of the fixing belt 20 which are not in contact with the printing medium P may be only slightly reduced or not reduced at all. Accordingly, if printing media P with low width dimensions, such as envelopes, are continuously fused, the temperature of opposite ends of the fixing belt 20 and/or the pressing roller 10 may increase, because these opposite ends of the fixing belt 20 and/or the pressing roller 10 are not in contact with the printing media P. As a result, the fixing belt 20 or the pressing roller 10 may become overheated and as a result may be damaged or broken.
Additionally, if a printing medium P with a high width dimension, such as an 8.5″×11″ sheet of paper, is fused after the printing media P with low width dimensions are continuously fused, a “hot offset” may occur, in which the developer fused at the opposite sides of the overheated fixing belt 20 adheres to the fixing belt 20.
In order to solve the above problems, when a printing medium P with a low width dimension is fused, a method is generally used in which the printing medium P is fused slowly so that the amount of heat generated at the heating member 50 is reduced throughout to reduce variation in temperature distribution. However, many image forming apparatuses have high-speed printing capabilities, so it is inefficient and difficult to use the method for slowly fusing a printing medium P with a low width dimension.